Many fireworks, especially those _____ are pushed into bamboo sticks, can lead to blindness or other injuries.
1. In that sentence, 'put in' is more authentic English than "pushed into".
2. "that" is the best answer. I would not use "which" in a sentence like that. "which" is often used when there is a choice involved, as in the Which I added above. And I would use "Which" there.
3. One could just simply say 'Many fireworks, especially those put in bamboo sticks,...'.
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